fix: separate beam search scoring for solution detection vs ranking

- Keep solution detection independent of op_scores priors
- Low/zero prior operations can still be found as valid solutions
- op_scores now only influence ranking/ordering, not filtering
- Add comprehensive tests demonstrating the fix works
- Fixes P1 issue where valid programs were dropped due to low priors

Closes: Programs with correct transformations using low-prior operations
are now properly found as solutions while preserving ranking behavior.

Author: tylerbessire